1. Field of the Invention
The present invention relates to a glow plug electrification control apparatus for controlling supply of electric current to a glow plug that assists startup of an internal combustion engine, and to a glow plug electrification control system using the same.
2. Description of the Related Art
In general, a glow plug has a resistance heater which is caused to generate heat upon supply of electric current thereto. The glow plug is configured such that a resistance heater is attached to a metallic shell, and is attached to the engine block of a diesel engine such that the distal end of the resistance heater is located within a combustion chamber.
A glow plug electrification control apparatus has been known as an apparatus for controlling supply of electric current to such a glow plug. Since such a glow plug has a relatively high resistance, a conventional glow plug electrification control apparatus is configured as follows. When a key switch is turned to an ON position, a switch (switching element) between a battery and the glow plug is maintained ON so as to supply a large current to the glow plug and raise the temperature of the heat generation section to a first target temperature (e.g., 1300° C.) which is sufficiently high for starting the engine. Such a step is generally called “pre glow” or a “pre glow step.” A glow plug capable of quick heating can raise the temperature of its heat generation section to the first target temperature within a few seconds (see e.g., Japanese Patent Application Laid-Open (kokai) No. S56-129763).
In recent years, a glow plug of a quick temperature raising type has been developed which can raise the temperature of its heat generation section to 1300° C. or higher (e.g., 1300° C.) within about 2 seconds, by further reducing of the resistance of the heat generation section, which enables a large current to flow through the heat generation section.
In a known control method performed while the temperature of the glow plug rises, the amount of cumulative power supplied to the glow plug is controlled so as to raise the temperature of the glow plug to a sufficiently high temperature without being affected by the battery voltage and so as to prevent excessive temperature rise. Specifically, voltage applied to the glow plug during the temperature rise and current flowing through the glow plug during the temperature rise are measured; electric power supplied to the glow plug is calculated and integrated so as to calculate the cumulative amount of electric power; and the temperature of the glow plug is raised until the cumulative amount of electric power reaches a predetermined value (see e.g., Japanese Patent Application Laid-Open (kokai) No. S60-67775).
Moreover, in a known technique, after the temperature of the heat generation section has been raised, the temperature of the heat generation section (heater temperature) is maintained in order to assist startup of an engine, stabilize operation of the engine after the startup, and reduce emissions (see e.g., Japanese Patent Application Laid-Open (kokai) No. 2004-44580). Specifically, this document describes that, in order to maintain constant the heater temperature of a glow plug whose resistance has a positive correlation with the heater temperature, the resistance of the glow plug is controlled such that it coincides with a target resistance. When such a control is performed, even when a disturbance (swirl or the like) arises, the heater temperature is readily maintained constant.
Notably, the resistance of the glow plug to be controlled includes not only the resistance of the heat generation section, but also the resistances of other members of the glow plug which form a path for supplying electricity to the heat generation section, and the resistance of a lead wire (wire harness) for supplying electric current to the glow plug.
3. Problem to be Solved by the Invention
However, even glow plugs of the same part number, which are industrially handled as the same part and are considered to have the same performance, show variations in the resistances of the heat generation sections, and thus show variations in their respective resistances.
Accordingly, when a battery voltage is applied via a switching element to a glow plug having a relatively low overall resistance because of a relatively low resistance of the heat generation section, a relatively large amount of current flows through the glow plug. As a result, the temperature rises quickly, so that the glow plug reaches a high temperature within a short period of time, and the cumulative amount of electric power supplied to the glow plug reaches a predetermined value within a short period of time. In addition, since the temperature of the glow plug rises within a short period of time, the amount of heat which escapes from the glow plug to an engine head or the like during the temperature rise decreases. Thus, the low-resistance heat generation section reaches a higher temperature, as compared with a glow plug which is high in resistance, even when the same cumulative electric power is supplied.
Further, when the resistance of the glow plug, including the lead wire, is then controlled to match the resistance to a target resistance, to thereby maintain the heater temperature, a relatively large amount of current is supplied to the glow plug so as to greatly increase the resistance. Therefore, the heater temperature is maintained at a relatively high temperature.
Meanwhile, in the case where the resistance of the glow plug, including that of the lead wire, is relatively large, a relatively small amount of current flows through the glow plug upon application of the battery voltage through the switching element. As a result, the speed of temperature rise is low, so that the glow plug requires a long period of time to reach a high temperature, and a long period of time is required for the amount of electric power supplied to the glow plug to reach the predetermined value. In addition, since the glow plug requires a long period of time to reach a high temperature, a larger amount of heat escapes from the glow plug to the engine head or the like during the temperature rise. As a result, the heater temperature can reach only a relatively low temperature, as compared with a glow plug which is low in resistance, even when the same electric power is supplied.
Further, when the resistance is then controlled so as to render the resistance coincident with the target resistance, to thereby maintain the heater temperature, a relatively small amount of current is supplied to the glow plug so as to prevent a great increase in the resistance. Therefore, the heater temperature is maintained at a relatively low temperature.
That is, due to variations in resistance among glow plugs (heat generation sections), variation arises not only in the temperature rising time but also in the temperature which the glow plugs can reach and in the heater temperature which is maintained through resistance control.
For example, in the case where the resistance of the glow plug, including the lead wire, is controlled to a predetermined fixed target resistance as described above, although the heater temperature of the glow plug can be maintained constant, a variation arises in the value itself of the heater temperature of the glow plug. In some cases, the variation in the heater temperature reaches several tens of deg C. to 200 deg C.
As described above, due to variations in the resistance of the glow plug, various problems arise, such as variation in engine startability and variation in ignitability immediately after startup.